The invention relates to a method of determining current positioning data of a machining tool and to an apparatus for this purpose and is to be used in particular in the production of ready-to-use tooth-restoration fitting bodies by means of grinding instruments in dental CAD/CAM grinding machines.
Grinding pencils having a surface of defined shape are especially suitable for the form-grinding of ceramic blanks. In an automated production process, it is necessary in this case to measure the position and dimension of the grinding pencil before each operation, but at least after changing the grinding pencil. This measuring can be achieved by feeling for a workpiece of known size. The feeling operation essentially comprises a grinding pencil rotating at low speed and a movement of the grinding pencil toward the workpiece or a movement of the workpiece toward the grinding pencil. The feeling operation is ended by the frictional contact between workpiece and grinding pencil, if the frictional contact has reduced the rotary speed of the grinding pencil to zero.
DE 40 30 175 C2 discloses a method of calibrating a motor-driven tool, which can be moved by means of a feed device toward and away from a workpiece to be machined, with regard to the workpiece or a holder accommodating the workpiece, and an apparatus for carrying out the method.
It has been found that the method described there reaches its limits when the shape of the frictional surfaces becomes extremely undefined or small, which, in particular in the case of grinding pencils narrowing toward the tip, can lead to considerable positioning inaccuracies during the axial feeling with the tip.
The object of the invention is to determine the exact linear positioning data of a tool having any desired tip geometry.
The machining tool according to the invention is moved in a slowly rotating manner toward a workpiece slowly rotating perpendicularly to the feed direction and having known geometry. If the tool strikes the workpiece, the rotation of the tool is stopped by the frictional load between workpiece and tool. In addition to the frictional load, a tangential force acts on the tip of the machining tool on account of the workpiece rotation, and this tangential force leads to increased frictional forces in the bearing arrangement of the drive shaft of the tool, so that the position in the feed direction can be determined in a considerably more precise manner.
It has been found that the penetration depth and in particular the variance of the penetration depth of the tool in the workpiece until the drive braked by the frictional forces comes to a stop can be markedly reduced in the case of certain tip geometries, which is immediately reflected in the accuracy of the determination of the position.
Advantageous developments are specified in the subclaims.
In addition, the present invention includes an apparatus for carrying out the method according to the invention.
In the case of the apparatus, there is a feed device with which the tool can be moved toward and away from a workpiece. Furthermore, the apparatus contains a first spindle for accommodating the workpiece, at least one further spindle for accommodating in each case at least one machining tool which can be set in rotation and has an end machining surface, the spindles being arranged and mounted in such a way that the machining tool and workpiece can be moved toward and away from one another for the purpose of material removal at the workpiece. Furthermore, the apparatus also contains drive motors for adjusting the spindles and for the drive of the machining tool and at least one reference surface which is arranged on the workpiece, on the work holder or on the clamping device and against which the end machining surface of the machining tool travels, a signal establishing the feed path of the tool being produced upon contact with the reference surface, which signal is used for determining the starting position of the machining tool. In addition, drive means are provided for moving the reference surface along the end machining surface of the machining tool during the contact produced by the feed, and a contact surface is formed with the reference surface by the end machining surface of the tool, the contact surface being asymmetrical and having direction components both in the feed direction and in the direction radial thereto, although only over a part of the circumference.
Advantageously, the tool has a narrowing tip and is in this case designed in particular as a spherical grinding pencil or tapered grinding pencil with tip.